I'm not sure what you mean. We can prove that a parachute allows people to survive jumping out of an airplane with limited injuries. They are effective.
For example, the fact that the impact velocity of some random object is reduced doesn't mean it is reduced from a harmful range to a non-harmful range. It might be equally harmful or equally harmless either way, or there might be additional factors at play in the case of human parachutists that we haven't thought about yet.
We don't have enough data on people jumping out of planes without parachutes to conclude that the parachutes are actually working. They could just be superstition.
That's absurd. The mechanics of gravity, parachutes and jumping out of planes is well understood. Plenty of people have fallen from heights without parachutes, and we know what happens to them and why (acceleration of gravity and the force impact on their bodies). We also know how parachutes, used properly, mostly prevent that sort of harm (slow people down enough to prevent excess force when impacting the ground). There's nothing superstitious about it.
Yes, and in a similar manner we can know quite a lot about how people would behave in situations like the one in Milgram's experiment even if we won't ever do an exact replication.
Yes, BUT in similar situations people behave very differently than the ones in Milgram's experiment. This is the whole point of it being not replicable.
That this remains true is due to a) no one being allowed to repeat Milgram's experiment exactly and b) more general scientific funding bodies not allocating money for pure replications.
